Where it All Goes Down
Don Juan covers an awful lot of ground in this poem. DJ has to leave Spain because he has an affair with a woman whose husband wants to kill him. Then he travels to some Mediterranean island where he has a sexual relationship with a girl whose father sells him into slavery, which takes him to Turkey. But just wait a second, because Turkey is invaded by the Russian army and DJ follows them back to Russia. Finally, Russia's empress sends him to England as a sort of ambassador.
Yup, Byron wanted his poem to be as epic as possible, and that means sending the hero to all sorts of exotic locations. It makes total sense that Byron eventually brings DJ to England, though. After all, England is the main target of Byron's satire, as we see in a line like: "If Britain mourn her bleakness, we can tell her,/ The very best of vineyards is the cellar" (13.76). In other words, Byron mentions that Britain often has overcast weather that nobody seems to like. But he reminds them that the climate can be good for certain plants to grow in.
Earlier in the poem, Byron also mentions how southern climates tend to make people (like Don Juan) more sexually open than people from northern regions like England. As Byron writes in Canto V, "[In] sad reality,/ Their chastity in these unhappy climes/ Is not a thing of that astringent quality/ Which in the North prevents precocious crimes" (5.157). Byron is clearly being tongue-in-cheek here, since he is actually calling out the English by being so prudish and connecting their sexual coldness to the coldness of their climate.